Dermatology - 香港醫學組織聯會

VOL.15 NO.11 NOVEMBER 2010
Is Psoriasis a Cutaneous Disease or
Systemic Disease?
Dr. LY CHONG
MBBS(HK), FRCP(Lond, Edin, Glasg), FHKCP, FHKAM(Med)
Private Dermatologist
Psoriasis is a well known chronic, non-contagious skin
disorder since ancient times. The word “Psora” is a
Greek word meaning “To itch”. It is notorious for its
chronicity in its natural course and difficulties in the
management. Although great efforts have been tried in
medical researches for decades, it is still not curable and
its exact aetiology remains unknown.
Prevalence in Chinese
In Western literature, it is well reported that 1-3% of
the Caucasian population have psoriasis. Though it
is a common skin disease, the prevalence reported in
Chinese is lower than that of Caucasians. From the
limited resources available, it is estimated that psoriasis
occurs in about 0.1-0.3% of the Chinese population. 1
In 1984, a nation-wide screening of psoriasis had been
conducted in 24 regions (53 centres) of China, involving
a coverage of 6,617,917 population. About 11,393 cases of
psoriasis (0.123% among the population studied; range
from 0.3%) had been reported. Interestingly,
the prevalence is highest in the Northeast provinces
and lowest in the Southern ones where there is more
sunshine. It is also higher in cities than the villages.
In Hong Kong, the prevalence is estimated to be
approximately 0.3% of the total population. 2 It is the 5th
leading new cases of skin disorders in public dermatology
clinics, with more than 600 new cases annually.
Immuno-pathogenesis of Psoriasis
Despite its unknown aetiology, there have been
breakthroughs in the understanding of the immunopathogenesis
of psoriasis in recent years. It is now
almost certain that psoriasis is a T-lymphocyte mediated
inflammatory dermatosis with hyper-proliferation
of keratinocytes in genetically predisposed subjects
(Diagram 1). It is regarded as one form of immunemediated
inflammatory diseases (IMID), which is a term
designated for organ-specific diseases in which cells and
cytokines of the adaptive immune system cause tissue
inflammation or destruction.
Until recent years, it was believed that IMID was
either mediated by Th1 T-cells (which is stimulated
by IL-12) or TH2 T-cells. The former subset includes
psoriasis, rheumatoid arthritis, multiple sclerosis
and inflammatory bowel diseases, while the latter
subset includes atopic dermatitis and asthma. A new
Medical Bulletin
Dr. LY CHONG
pathogenic concept in IMID however is developed upon
the discovery of a new T-cell lineage in 2005. This new
cell lineage is called Th17 T-cell, which is defined by the
production of IL-17, and stimulated by IL-23. This subset
likely includes psoriasis, rheumatoid arthritis, multiple
sclerosis and inflammatory bowel diseases. IL-12 and IL-
23 are structurally related with a common 40kD subunit
(p40), which leads to the development of a new group of
biologics called anti-P40 (anti-IL12/23) that blocks both
the Th1 and Th17 pathways.
Diagram 1. Postulated Immuno-pathogenesis of Psoriasis
Co-morbidities of Psoriasis
The traditional belief about psoriasis is that it is a
cutaneous disease without visceral involvements, albeit
10-30% of the patients have joint involvements. This
concept is challenged in recent few years when more and
more systemic co-morbidities had been reported. When
the term “psoriasis and co-morbidities” is searched in
Medline, more than 200 articles can be retrieved over
the past two decades. It is one of the hottest research
topics in dermatology in the past 5 years, as shown by
the numerous publications from different countries and
different indexed journals. The possible co-morbidities
of psoriasis reported in literatures are summarised in
Table 1. Three important areas which may have impacts
on medical health are worth mentioning here.
Cardiovascular diseases (hypertension, myocardial infarction),
cerebrovascular and peripheral vascular diseases
Metabolic syndrome (obesity, diabetes mellitus)
Non-alcoholic fatty liver
Autoimmune diseases (Crohn’s disease, ulcerative colitis, multiple sclerosis)
Lymphoma, melanoma, non-melanoma skin cancers
Depression, suicide
Smoking, alcoholism
Osteoporosis
Table 1. Possible co-morbidities of psoriasis reported in literatures
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